1. Field of the Invention
This disclosure relates to a wireless communication system, and more particularly, to a method of generating pseudo random sequence in a broadband wireless communication system and an apparatus for generating pseudo random sequence using the method.
2. Discussion of the Background
Scrambling code is used for identifying mobile station (user equipment or UE)-specific and cell (base station)-specific identification information in a wireless communication system. Generally, scrambling code identifies a base station in a downlink and identifies a mobile station in an uplink. This type of scrambling code randomizes or decreases the interference in a mobile station or a mobile station by other mobile station or cell, and may be used as one of the methods for increasing the cell capacity.
In the case of a 3GPP Long Term Evolution (LTE) system, the scrambling code is commonly applied to each physical channel and each physical signal of an uplink and a downlink. For example, in the process of an uplink physical channel in the 3GPP LTE system, each bit which passed through channel coding is scrambled with signs generated by a scrambling generator bit by bit through a modulo 2 operation. The scrambled bits are input to a modulation mapper to be mapped to complex-valued symbols by modulation methods such as Quadrature Phase Shift Keying (QPSK), 16 Quadrature Amplitude Modulation (16QAM), and 64 Quadrature Amplitude Modulation (64QAM). In the process of a downlink physical channel in the 3GPP LTE system, similarly to the uplink, bits which passed through channel coding and were inputted as code words are scrambled and inputted to the modulation mapper.
In general, the aforementioned scrambling code is generated on the basis of a pseudo-random sequence having good correlation characteristics. A well-known pseudo-random sequence may include an m-sequence, a Gordon-Mills-Welch (GMW) sequence, a Legendre sequence, or the like. The m-sequence may be converted into a primitive polynomial of degree m over GF(2), and implemented by using a Linear Feedback Shift Register (LFSR).
The pseudo-random sequence such as the m-sequence has optimal periodic autocorrelations. However, since the size of only a single m-sequence is 1, there is a limit to using the pseudo-random sequence as scrambling code requiring different random sequences having good cross-correlations (the maximum cross-correlation value is low and the number of kinds of cross-correlations is small). Therefore, in general, by mathematically connecting pseudo-random sequences, different pseudo-random sequences of size M are generated to be used as scrambling code. Particularly, one of the most widely used methods in a wireless communication system is a method of mathematically connecting two m-sequences and generating scrambling code using a Gold sequence generated as a result.
Recently, with the development of a wireless communication system, in order to identify more pieces of mobile station-specific and cell-specific system information, different scrambling code groups having a larger size are needed. In a 3GPP Wideband CDMA (3GPP WCDMA) system developed on the basis of the Global System for Mobile communication (GSM), which is one of the most prominent standards among standards of the third-generation International Mobile Telecommunications-2000 (IMT-2000), scrambling code for identifying specific system information of 25 bits by using a Gold sequence in the case of m=25, is used.
In a 3.9G (Pre-4G) LTE system developed from the 3GPP WCDMA, in order to identify more sufficient pieces of information than the early standardization process, a method of generating scrambling code having a very large size was proposed. However, in the method of identifying mobile station-specific and cell-specific system information of 40 to 50 bits or more by using the Gold sequence in the case of m=40 or m=50 or more as the proposed scrambling code, there is a problem in that hardware complexity increases due to blocks (block or box) in a number of LFSRs connected in series.
In consideration of this, in the early standard TS36.211-8.1.0 of a physical layer part of the 3GPP LTE, in order to decrease the hardware complexity, it was agreed that mobile station-specific and cell-specific system information necessary for calculation was limited to 33 bits, and identified by scrambling code generated through the Gold sequence in the case of m=33. However, in the case of m=33, in consideration of hardware (HW) based on the 32-bit architecture, there still is a problem in terms of hardware complexity. Accordingly, a Gold sequence of m=32 or less has been considered. In the case of m=32, since m is a multiple of 4, the Gold sequence does not exist. Therefore, it has been finally determined that mobile station-specific and cell-specific system information is identified by scrambling code generated from a Gold sequence in the case of m=31 (TS36.211-8.3.0).
Large sets of scrambling codes are needed to identify more pieces of information. However, in consideration of hardware complexity of a current system, the standard is determined such that a minimum number of pieces of information are identified. For this, a method of generating scrambling code using a Gold sequence in the case of m=31 has been adopted. However, in a next-generation broadband wireless communication system which can require a number of different scrambling codes as a frequency band is broadened and a cell radius of, such as, a pico cell and a femto cell is reduced, the number of bits which is limited to identify mobile station-specific and cell-specific system information in a related art has to be increased. For this, instead of considering a 32-bit structure of hardware in the related art, a Gold sequence in the case where m is increased is considered while accepting hardware complexity.
Considering three factors which have an affect on the performance of scrambling codes, that is, size, maximum cross-correlation value, and period, since the Gold sequence in the case of m=31 is sufficient as a period (length) of scrambling code in an existing wireless communication system, a technique for maximizing size while reducing or maintaining the maximum cross-relation value is proposed.